The spectrum of the planetary nebula IC 418

Abstract

A detailed high-spectral-resolution study of the spectrum of IC 418 is made for the region 3650 to 10050 Â, using the Hamilton echelle spectrograph of Lick Observatory, and of the UV spectral region with archival International Ultraviolet Explorer (IUE) data. From high-resolution images in both the near-and mid-infrared, Hora et al. (1993) showed that IC 418 probably has a compact shell interior to the detached, well-known, main shell emission. If one assumes a black body or Hubeny (or standard LTE) model atmosphere energy distribution, it does not appear possible to construct a fully satisfactory nebula model that will simultaneously represent the Hß flux, the [O m]5007/H/3 ratio, and the scale of this planetary nebula (PN). Fortunately, IUE and IR data supply information on ions in addition to those optically observed so that the chemical composition can be reasonably well established by summing over concentrations of observed ions. In spite of the fact that IC 418 is carbon rich in sense of having a C/O ratio exceeding the solar value, it is a "metal-poor" object. Possibly it resembles IC 4997 but in a more advanced evolutionary phase. The central star is variable and has a strong wind.

Identification and Characterization of Faint Emission Lines in the Spectrum of the Planetary Nebula IC 418

“…We used 20 pairs of hydrogen Balmer and Paschen lines arising from the same upper levels to find a reddening coefficient c Hβ = 0.34 ± 0.05. This value fits in the middle of the range of other published values including 0.25 from Harrington et al (1980), 0.29 from Mendez (1989), 0.21 from Hyung, Aller, & Feibelman (1994), 0.34 determined by Shaw & Dufour (1995) in their re-analysis of the Hyung, Aller, & Feibelman (1994) data, and 0.14 from Henry, Kwitter, & Bates (2000). This wide scatter may be due to differential reddening across the nebula and the differing regions imaged, and/or to the difficulty in determining an accurate reddening value due to differing qualities of flux calibration, and numbers and types of lines utilized in its calculation.…”

“…Although Hyung, Aller, & Feibelman (1994) speculates that some O II lines in IC 418 may be susceptible to continuum fluorescence, Grandi (1976) predicts only 20% contribution at most for multiplet V2, and much less for other of the observed multiplets. There is no evidence for continuum fluorescence excitation among any of the line profiles shown in Figure 6.…”

Abundance Determinations In HII Regions And Planetary Nebulae

“…The central star temperature is estimated to be 140 000 -180 000 K, the gas density is around 5 10 4 cm −3 . Kwitter & Henry (1996), optical + UV data, model c Keyes et al (1990), optical + UV data, model d Middlemass (1990), optical + UV + FIR data, model e Perinotto et al (1980), optical + UV data, empirical f Péquignot et al (1978), optical + UV + FIR data, model g Shields (1978), optical + UV + FIR data, model h Aller 1954, optical data, empirical Henry et al (2000), optical + UV data, model b Hyung et al (1994), opt echelle + UV + a few IR data, model c de Freitas Pacheco et al (1992), optical data, empirical d Gutierrez Moreno (1988), optical data, empirical e Aller & Czyzak (1983), optical data, hybrid method f Harrington et al (1980), optical + UV data, empirical g Barker (1978), optical data, empirical h Torres- Peimbert & Peimbert (1977), optical data, empirical with t 2 = 0.035 Table 6. Abundances in IC 418…”

Line Ratio Diagnostics Applicable to Astronomical Spectra in the 50–3000 Å Wavelength Region

“…Over the past 20 years, many such diagnostics have been developed for application to astronomical spectra, such as those of the solar transition region/corona (Dere 1978;Vernazza & Reeves 1978), and stellar and interstellar observations from the International Ultraviolet Explorer and Copernicus satellites (Hyung, Aller & Feibelman 1994;Jenkins & Shaya 1979;Jordan 1988 (HUT), have lead to a large increase in both the quality and quantity of astronomical UV and EUV observations, and there is clearly an urgent requirement for diagnostics which may be applied to the analysis of such data.…”